The present invention relates to a current control apparatus for controlling the magnitude of a current applied to an object of control, and more particularly to a current control apparatus which is used for controlling an actuator that produces a force corresponding to the magnitude of the current applied.
Electromagnetic proportional valves are used to control the flow rate of pressured oil fed to hydraulic actuators in hydraulic circuits of construction machines and the like. The degree to which such electromagnetic proportional valves are open is virtually proportional to the magnitude of the current applied from a controller to a solenoid attached to the electromagnetic proportional valve. The pressured oil is thus fed to the hydraulic actuator at a flow rate corresponding to the degree to which the valve is open.
In this case, the excitation current actually flowing through the solenoid coil is detected, and the controller controls the current applied to the solenoid so as to obtain a target current value, using the detected current value as the amount of the feedback.
That is, the controller computes at a prescribed time interval a duty corresponding to the target current value that is input at the prescribed time interval, and generates a pulse signal having this duty, which are applied to a driving transistor. When the driving transistor is actuated according to the pulse signal input, current is applied to the solenoid coil, resulting in that the aforementioned target current value is obtained.
However, in actual operations, as a period of applied electricity becomes longer and the temperature of oil, etc. is increased, the temperature of the solenoid coil becomes higher, and the direct current resistance of the coil, etc. is increased accordingly, which prevents current corresponding to the target current value from being applied to the coil and results in lower control precision.
In an effort to prevent the loss of precise control caused by such increases in the coil temperature, Japanese Patent Publication 62-59444 discloses technology that a current is allowed to flow through the solenoid at a constant level which does not trigger the hydraulic actuator when the operating lever is returned to the neutral position in controllers designed to provide the solenoid with a control target value (target current value) in response to the operation of the operating lever the correction coefficient is determined based on the value of the duty at this time and the filtered average detected current value, and the value of the duty is corrected using this correction coefficient.
Although there are no problems when the operating lever is in a neutral zone in the technology described in this patent publication, control errors are produced, even when the duty is corrected, in cases where the operating lever is moved from the neutral zone to full operation.
In the device described in Japanese Patent Publication 7-66299, the excitation current flowing through the solenoid coil is integrated by integration means in synchronization with a PWM pulse signal, and the duty is corrected on the basis of the control target value and the integrated value output from the integration means. This allows errors in control to be avoided when the operating lever is moved from the neutral zone to full operation in order to increase the excitation current to the coil.
In the devices described above, it is assumed that the control target value (target current value) is input to the controller in response to operation of the operating lever, and the control target value is not expected to fluctuate greatly.
However, in cases where the control target value input to the controller has been produced in response to a signal indicating the engine rpm, the considerable fluctuation in the engine rpm results in considerable fluctuation in the control target value at a prescribed time interval.
Such considerable fluctuation in the control target value at a prescribed time interval cannot be dealt with by the technology described in the aforementioned Japanese Patent Publication 62-59444, and control errors are still produced.
Further, when the technology described in the aforementioned Japanese Patent Publication 7-66299 is applied to deal with the fluctuation, the realization of the technology is extremely difficult and control instability is incurred.
That is, the excitation current to the coil is integrated according to PWM pulse cycles in the device described above, resulting in the need for A/D conversion at high speed sampling within a PWM pulse cycle for high-precision integration of the excitation current. This requires a high-speed, high-precision A/D converter, which is extremely difficult to realize.
The control current also tends to be unstable for the following reasons.
1) As it is difficult to achieve the feedback of the integration within every two cycle, it is likely to generate lag and result in instability.
2) In the case of changing the control current value dictated on the basis of the correction coefficient that is to be determined, the correction coefficient is either too great or too little because the actual current cannot be immediately adjusted due to the inductance of the solenoid. This is produced by delays of 1 cycle or more for the same reason as in 1), again tending to result in instability.
As described above, a problem in the prior art is that current applied to the object of control such as a solenoid coil cannot be controlled with high precision in a stable manner in cases involving considerable fluctuation in the control target value at a prescribed time interval.
The present invention is intended to remedy such a drawback.
The main invention of the present invention is a current control apparatus having duty computing means for computing and outputting at a prescribed time interval a duty corresponding to a target current value input at the prescribed time interval, pulse signal generating means for generating a pulse signal having the duty output from the duty computing means, and an object of control to which electricity is supplied when the object is driven by the pulse signal generated by the pulse signal generating means, the current control apparatus comprising current value detecting means for detecting a current value applied to the object of control and for outputting the detected current value at a prescribed time interval; corrected current value computing means for computing and outputting at a prescribed time interval a present corrected current value, based on a preceding corrected current value and the current value presently output from the current value detecting means, so that the present corrected current value has a value intermediate between the preceding corrected current value and the current value presently output from the current value detecting means; and corrected duty computing means for computing and outputting at a prescribed time interval the present corrected duty, based on the preceding corrected duty and the duty presently output from the duty computing means, so that the present corrected duty has a value intermediate between the preceding corrected duty and the duty presently output from the duty computing means, wherein the duty computing means computes and outputs the duty based on the input target current value, the corrected current value output from the corrected current value computing means and the corrected duty output from the corrected duty computing means.
Because the duty computing means thus computes and outputs the duty based on the input target current value, the corrected current value output from the corrected current value computing means, and the corrected duty output from the corrected duty computing means, current coinciding with the target current value can flow through the object of control and control precision can be dramatically improved, despite the resistance of the control object and the changes in the voltage or the like from the power source applied thereto.
Further, the present corrected current value used to compute the duty output from the duty computing means is also computed by the corrected current value computing means at prescribed time interval based on the preceding corrected current value and the current value presently output from the current value detecting means, so that the present corrected current value is midway between the preceding corrected current value and the current value presently output from the current value detecting means, and the present corrected duty used to compute the duty output from the duty computing means is also computed by the corrected duty computing means at prescribed time interval based on the preceding corrected duty and the duty presently output from the duty computing means, so that the present corrected duty is midway between the preceding corrected duty and the duty presently output from the duty computing means. Therefore, the value of the duty with good response and follow-up performance can be computed and output, and control stability can be dramatically improved, despite considerable fluctuation in the target current values at a prescribed time interval.